1. Field of the Invention
The invention generally relates to data communication systems such as WLAN (Wireless Local Area Network) systems, and in particular to decision feedback equalizers that may be operated in receivers in such systems.
2. Description of the Related Art
A wireless local area network is a flexible data communications system implemented as an extension to or as an alternative for, a wired LAN. Using radio frequency or infrared technology, WLAN systems transmit and receive data over the air, minimizing the need for wired connections. Thus, WLAN systems combine data connectivity with user mobility.
Today, most WLAN systems use spread spectrum technology, a wide-band radio frequency technique developed for use in reliable and secure communication systems. The spread spectrum technology is designed to trade-off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems.
The standard defining and governing wireless local area networks that operate in the 2.4 GHz spectrum, is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to 802.11b that allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. This extension is backwards compatible.
Receivers in WLAN systems or other data communication systems usually apply some nonlinear equalization technique to reduce the effects of intersymbol interference and to compensate for the frequency response of the channel. One commonly applied technique is the use of a decision feedback equalizer (DFE) which is an adaptive filter that uses data decisions to adjust its taps to correct for frequency response impairment.
An example of a decision feedback equalizer is shown in FIG. 1. The depicted decision feedback equalizer consists of two filters, a feedforward filter 100 and a feedback filter 110. The input to the feedforward filter 100 is the received signal sequence of the receiver. The feedback filter 110 feeds the output data back so that the output data can be combined with the output sequence of the feedforward filter 100. There may be additionally a decision unit (not shown) that receives the output data and that is connected to the feedback filter 110 so that the feedback filter 110 actually receives its input from that decision unit.
While decision feedback filters in data communication receivers have been shown to effectively improve the signal quality and thus the overall system performance, conventionally technologies disadvantageously require complex and highly involved circuitry to perform their functions. For instance, high precision fixed point multiplications need to be performed, and the decision feedback equalizers require a large amount of multipliers and other arithmetic modules for this purpose. This leads not only to high development and manufacturing costs but also increases the chip area needed to implement such functions. Thus, the conventional techniques are disadvantageous by preventing the circuits from being scaled down in size and power consumption.